The present invention relates to an automatic gain control circuit (hereinafter called simply AGC circuit) in which an amplitude of an output signal is maintained constant despite variations in the amplitude of an input signal applied to the AGC circuit.
Among known AGC circuits, a circuit has been used in which the amplitude of an output signal is maintained constant without directly varying the gain of an amplifier amplifying an input signal applied to the circuit. To that end, a variable impedance circuit is utilized, in which a control signal is applied to its first input and an impedance between its second and third terminals is varied in response to the control signal. The output of an amplifier with a fixed gain is converted into D.C. level and applied to the first terminal of the variable impedance circuit as the control signal. The input signal for the amplifier is attenuated, before being applied to the amplifier, by the impedance between the second and third terminals of the variable impedance circuit. As the input signal amplitude is increased, the control signal derived from the amplifier varies the impedance of the variable impedance circuit to increase the amount of attenuation against the input signal amplitude. As the input signal amplitude becomes small, the impedance of the variable inpedance circuit is varied to decrease the attenuation amount of the input signal amplitude. Thus, the output signal amplitude can be made substantially constant independently of the variations in the input signal amplitude.
The variation in the impedance between the second and third terminals of the variable impedance serves to vary the input signal amplitude, but this variation should not change the input bias voltage of the amplifier. If the input bias voltage is changed, an intended amplification operation cannot be realized. Accordingly, the variable impedance circuit is normally connected to the input terminal of the amplifier through a capacitor to cut any D.C. component. However, the D.C.-cutting capacitor must have a large capacity, and therefore, it cannot be formed within a monolithic semiconductor integrated circuit. For this reason, the D.C.-cutting capacitor is connected to external lead terminals of an integrated circuit device as an externally provided part executing a circuit operation jointly with the integrated circuit. An increase in the number of the externally provided parts causes an increase in external lead terminals of the integrated circuit device, resulting in increase in a cost of the AGC integrated circuit device as well as a degradation of the reliability of the electrical and mechanical connection with the externally provided parts.
One possible structure which avoids the need for the D.C.-cutting capacitor to D.C.-couple the variable impedance circuit to the input terminal of the amplifier without varying the input bias voltage is as follows. The second and third terminals of the variable impedance circuit are respectively D.C.-connected to the input terminal of the amplifier and reference voltage generation circuit generating a reference voltage which defines an input bias of the amplifier. A means for supplying the input bias voltage is connected in parallel with the variable impedance circuit between the reference voltage generation circuit and the input terminal of the amplifier, and the current flowing therethrough is so small that the voltages at the second and third terminals of the variable impedance circuit are substantially equal to each other even if an impedance variation arises between the second and third terminals. Accordingly, the input bias voltage supplied to the input terminal of the amplifier is not substantially affected by the impedance variation of the variable impedance circuit. Since the input signal is an A.C. signal, it is attenuated in accordance with the impedance between the second and third terminals of the variable impedance circuit. In this way, the variable impedance circuit can be D.C.-coupled to the input terminal of the amplifier.
However, the output impedance of the reference voltage generation circuit is not zero. Rather, it has a certain finite value. Consequently, the input signal cannot be sufficiently attenuated even when the impedance between the second and third terminals of the variable impedance circuit takes the lowest value in response to the control signal applied to its first terminal. In other words, the gain control range is narrowed.